For many years, designers of aircraft have recognized the necessity of providing a continuous reading of the air speed of the aircraft during its operation. There have been a number of different approaches to the measurement of air speed and of the other performance data that are relevant to the operation of the aircraft. The traditional approach to the measurement of air speed was the so-called "pitot-static tube", which is a device usually mounted on the wing of the aircraft and which samples and compares the dynamic and static pressures of the air at the location of the pitot-static tube. With the advent of high- performance aircraft, the pitot-static tube was no longer satisfactory as a device for measuring true air speed. Moreover, it was not capable of accumulating data useful in evaluating other quantities such as the angle of attack and the angle of sideslip (when these parameters become large) of the aircraft.
Especially after the advent of supersonic aircraft, which generate a shock wave that surrounds the aircraft in its flight, it became necessary to take a totally new approach to measuring the motion of the aircraft with respect to the air in which it is flying.
One modern approach depends upon the use of the laser as a source of optical radiation which may be directed from an aircraft into the surrounding air and which may be scattered by tiny aerosol particles having a typical size of a few micrometers or less. Another approach has been to direct a beam of optical radiation from a laser into the atmosphere in an attempt to excite the molecules of oxygen and nitrogen in the atmosphere to cause them to fluoresce, giving off light which could then be detected aboard the aircraft and which would give certain information concerning the relative motion between the aircraft and those oxygen and nitrogen molecules. A disclosure of this nature appears in U.S. Pat. No. 4,483,614, issued on Nov. 20, 1984 to Philip L. Rogers, and entitled "Optical Air Data Measurement System". In that reference, light which is backscattered from aerosol particles in the air is compared with fluorescent light radiation given off by molecular particles as a result of excitation by the light flux from the laser aboard the aircraft. Comparison of the two types of light produces a fringe pattern which is meaningful in velocimetry. The aforementioned data-measurement system would, in practice, be very complicated.
One particular factor which has emphasized the need for a new type of velocimeter has been the development of high-performance aircraft characterized by negative stability. Such inherently unstable aircraft must be controlled continuously at all times during their flight, or they will go completely out of control and crash. The requirement for complete and continuous control has been made even more difficult to satisfy because fighter aircraft must be fully aerobatic in order to carry out the violent maneuvers of aerial combat. In order to satisfy that requirement, a data rate of about 60 Hertz is regarded as necessary. The problem of providing a sufficient data rate is exacerbated at high altitudes where the density of particles suitable for backscattering optical radiation is much less than at low altitudes. Still further, the problem becomes extremely difficult when the aircraft is flying near its stall speed or when it is moving with large components of sidewise or vertical motion, commonly referred to as "sideslip" or "angle of attack" respectively. It has not been possible to overcome these difficulties by means of prior-art techniques typified by the aforementioned Rogers patent, which probably represents the most advanced prior art up to the time of our invention.